Yes, this is by design (Find&Replace only operates on data that actually exists in the pattern - the default volume is just displayed for convenience but does not exist as a volume command). To "find" and replace default volumes, do this:

• Find note: any (plus whatever your other search criteria might be - e.g. instrument number)
• Find volume effect: none
• Replace with volume effect: v
• Replace with volume: Whatever value (or percentage) you like.

Ahh, understood; works a charm. Thank you. :) OpenMPT is most excellent. ^-^